1. Field of the Invention
The present invention relates to a novel photopolymerization initiator and a photosensitive composition employing the same, and particularly to a novel photopolymerization initiator comprising a borate salt, and a photosenstive composition employing the same.
2. Related Background Art
A diversity of compounds are known as photopolymerization initiators: such as azo compounds, organic sulfur compounds, carbonyl compounds, and oxidation-reduction systems employing ferric ion. At present, most generally used ones are organic carbonyl compounds which are advantageous in that the sensivity thereof is relatively high, and the compounds are stable, and easily synthesized. The compounds, however, are limited in the application wavelengths up to 450 nm at the longest because the photosensitivity wavelength range thereof is equal to the absorption wavelength range of the compounds per se.
On the other hand, recently a combination of a pyrylium dye with a peroxide has enlarged the photosensitive range to long wavelength side. The combination, however, has a disadvantage of low storage stability which causes a short shelf life.
The need for photopolymerization initiators sensitive to longer wavelengths have resulted from the recent remarkable progress in laser technology. That is, the need has come from the desire to apply laser beams to recording processes such as resist patterning and photoengraving instead of using conventionally employed short wavelength light such as ultraviolet light. In particular, strongly desired are use of inexpensive and miniaturized semiconductor laser. The laser beam shorter than 650 nm, however, is considered not to be attainable. Under such circumstances, photopolymerization initiators have been extensively investigated which are sensitive to light of wavelength of 650 nm or longer.
Japanese Patent Laid-open Publication No. 62-143044 (1987), for example, discloses a photopolymerization initiator comprising a borate salt of a dye and having sensitivity at 500 nm or longer.
The initiator has disadvantages in that the synthesis of the compound is difficult and that the compound per se and the photosensitive composition derived therefrom are both unstable.